The present invention relates to gas discharge display panels with internal memory and, more particularly, to such display panels which operate in a DC mode.
Gas discharge display panels in which two orthogonal sets of conductors are disposed on opposite sides of an ionizable gas are well known in the art. In such devices, a potential applied to one of the anodes and one of the cathodes will result in the breakdown of the gas at the intersection of those electrodes, and the resulting gas discharge will emit light in the visible region of the spectrum.
In AC gas discharge display panels, the electrodes are isolated from the gas by a dielectric. This dielectric capacitor acts as the memory element of the cells and also provides the current limiting mechanism. During each half-cycle of the AC excitation signal, a wall charge will build up on the surface of the dielectric in contact with the gas, and this wall charge will oppose the drive signal, permitting use of lower voltage signals to sustain or maintain the discharge. This is advantageous in an AC gas discharge display panel since the wall charge will rapidly extinguish the gas discharge and assist in breaking down the gas during the next half-cycle of the AC signal. Since each breakdown during each half-cycle of operation produces light emission from the selected cell or cells, a flicker-free display can be achieved by operating the display at a relatively high frequency, e.g., 30-50 kilocycles. A disadvantage of AC gas discharge display panels is that the AC drive signal generation systems are quite expensive and the brightness and efficiency are low.
An alternative to the AC gas discharge display panel is a DC gas discharge panel which, like the AC panel, consists of two sets of orthogonally arranged conductors enclosing an ionizable gas. In conventional DC operated gas discharge display panels, the metal electrodes are in direct contact with the discharge. Therefore, the cathodes are subjected to constant bombardment by gas ions during DC operation. These gas ions may have sufficient kinetic energy to sputter atoms from the cathode surface. While many of the sputtered atoms will be deflected by collisions with gas atoms, some will escape collision with the gas atoms and be deposited on other surfaces within the device. Continued sputtering will result in the production of electrical leakage paths between conductors and in the trapping of inert gas by sputtered deposits, with consequent loss of gas pressure. These sputtering effects will restult in a decrease in the usable life of the device and they will also make cell switching more difficult.
Certain techniques have been proposed to control sputtering of the cathodes in a DC gas-discharge display panel, but none have proven satisfactory. If a protective layer overlying the cathodes is employed in a DC panel, such a layer cannot be a dielectric protective layer, since a dielectric will isolate the gas discharge cell from the DC excitation voltage. In contrast to the AC panel, in which a surface charge build-up is desirable in order to aid in extinguishing the discharge and in causing gas breakdown during the next half cycle of operation, a surface charge build-up in a DC operated panel will decrease the effective potential applied to the gas until the net voltage falls below the minimum voltage required to sustain a gas discharge, at which time the cell will turn "off".
A somewhat similar problem has also been recognized in AC gas discharge display panels. In AC panels, the dielectric glass layer overlying the metal electrodes and isolating them from the gas can become dissociated and sputtered due to ion bombardment from the discharge. Therefore, the dielectric glass layer in AC panels is covered with a protective refractory layer made of a material having a high binding energy such as magnesium oxide. In DC gas discharge display panels, on the other hand, a protective dielectric layer of a high binding energy metal oxide such as magnesium oxide overlying the metal cathodes cannot be employed to correct cathode sputtering, since any surface charge build-up is undesirable in DC operation.
For DC gas discharge display panels operated in a storage mode, a current limiting element, usually a resistor, must be used in series with each cell to increase the overall impedance of the cell, since the impedance of the cell due to discharge alone is generally low. This gives the cells internal memory and, once the cells are switched on, the discharges can be sustained by a fixed DC voltage until erasure is required. Certain proposals have been made for producing internal resistors in DC panels with internal memory, but none have proven satisfactory.